Tire Size Box

325/40R18 vs 325/45R18

325/45R18 compared with 325/40R18

+4.5% diameter, 0.0" width

325/45R18 is 29.52" tall vs 28.24". More than the 3% rule of thumb, so check carefully before switching.

28.2"18" rim5.1"12.8" wide
325/40R18 (solid) compared with 325/45R18 (dashed outline), drawn to scale

With 325/45R18 tires, when your speedometer says 60 mph you are really going

62.7 mph

325/45R18 is +1.28 inches (+4.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R18325/45R18Difference%
Overall diameter28.24"29.52"+1.28"+4.5%
Section width12.80"12.80"0.00"0.0%
Sidewall height5.12"5.76"+0.64"+12.5%
Circumference88.71"92.73"+4.02"+4.5%
Revolutions per mile714.3683.3−31.0−4.3%

Speedometer difference

If your vehicle came with 325/40R18 and you fit 325/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.8 mph+1.8 mph
50 mph52.3 mph+2.3 mph
60 mph62.7 mph+2.7 mph
65 mph67.9 mph+2.9 mph
70 mph73.2 mph+3.2 mph
75 mph78.4 mph+3.4 mph
80 mph83.6 mph+3.6 mph

Each size on its own

Questions

Which is bigger, 325/40R18 or 325/45R18?
325/45R18 is taller by 1.28 inches (29.52" vs 28.24"). 325/40R18 is the same width.
Can I replace 325/40R18 with 325/45R18?
It is not recommended without checking carefully. The diameter difference is +4.5%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 325/45R18 change my speedometer?
When your speedometer reads 60 mph with 325/45R18 fitted (calibrated for 325/40R18), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.5%.
Will 325/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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